Exporting from Gridview to Excel pdf article

Looking at Mudassar's great article on exporting the GridView to various file types.  I have got the PDF working great and also want to add Excel.  When I use the code I get a message that the file I am trying to open is a different file format from the extension I am trying to open.  If i click OK it opens and alls is fine.  Trying to get rid of this errors so customers and sales staff doesn't see it.  Excel is version 2010.